Quadrature amplitude modulation (QAM) has conventionally been known as a modulating scheme to transmit data by adjusting the amplitudes and phases of two carrier waves that are independent from each other. For example, a modulator is known as a QAM modulator, that is configured to: cause plural Mach-Zehnder modulators to execute multi-value (for example, four-value) phase modulation; and couple the acquired optical signals (see, e.g., Japanese Laid-Open Patent Publication No. 2009-244682).
However, according to the conventional technique, the Mach-Zehnder modulator executing the multi-value phase modulation has many components to be controlled such as a π/2 shifter and a bias supply unit. Therefore, a problem arises that the control of the Mach-Zehnder modulator is complicated.